1. Field of the Invention
The present invention relates to circuitry for maximizing the output power of a power supply.
2. Description of the Prior Art
The power provided by a power supply is the product of the voltage across the output terminals and the current appearing at those terminals.
Typically, the output voltage of the power supply decreases to a greater or lesser extent as the current increases to provide a "droop" to the graphic representation of the relationship between the output voltage and current. See FIG. 1. The graph of output voltage and current will change both in magnitude and shape as the result of independent variables applied to the power supply. For example, the output voltage of a photovoltaic solar cell is a function of temperature while the current is a function of illumination. Power supplies energized by other types of geophysical energy, as well as by other means, exhibit other variations in voltage and current output characteristics.
It is often desired to maximize the power output of a power supply. For example, in a power supply with a finite amount of power, such as a battery or fuel cell, one may wish to extract the stored power at a maximum rate. To accomplish output power maximization, the magnitude of one of output voltage or output current is usually made variable and adjusted to a magnitude which maximizes the output power. For example, in the case of a battery, the current is usually varied. Given the droop in the voltage-current relationship of a power supply, the relationship of output power to the output current resembles a bell-shaped curve. At high voltage and low current, output power is low. As the output current increases, output power similarly increases up to the point at which the voltage begins to droop whereupon the power output decreases. See FIG. 2. To maximize power, an output current is thus selected which corresponds to a value Im.sub.1 which provides maximum power output.
Time varying changes in the relationship of output voltage to output current, because of independent variables related to the power supply changes the relationship of output power to the output current or voltage as shown by the dotted lines in FIG. 2 which graphs output power with respect to output current. The current level which provides a maximum power is altered, as from Im.sub.1 to Im.sub.2.
The changes in relationship of output power to output current or voltage require that any workable scheme of power maximization provide regulating circuitry for establishing the output current or voltage so as to produce maximum power output and for adjusting magnitude of the variable quantity to a magnitude which produces that maximum power output.
The design and operation of such regulating circuit is rendered difficult by the absence of any readily identifiable phenomenon, such as a polarity reversal, to identify when maximum power output conditions exist. For this reason, a technique known as "dithering" is restored to. In this technique, minor variations--increases and decreases--are made to the output current or voltage and the corresponding variations in power ascertained, as by sensing phase relationships between the power and the output current or voltage. If for example, current is used as the variable quantity and the current is at a value corresponding to maximum power output, both increases and decreases in the current will result in decreases in output power. If the current is at some other value, for example less than that required for maximum power conditions, decreases in the current will result in decreases in power and vice versa, indicating that the output current is less than a value corresponding to maximum power output. The regulator then increases the current until the maximum power output condition is obtained.
Obtaining power maximization by dithering presently suffers several drawbacks. One is its theoretical and practical inefficiency due to the fact that under conditions other than maximum power conditions, the current or voltage is being adjusted in a direction opposite to that required for maximum power at least half of the time. Further, there is a limit to the rate at which the magnitude of the current or voltage can be altered in moving the maximum power condition. This rate limitation is typically 1/5th, 1/10th, or some other fraction of the dithering rate and slows the response of the control in establishing the maximum power output condition.